1. Field of the Invention
The present invention relates to a spectrometer in which the wavelength resolution is improved without being affected by the pitch of a photodiode array.
2. Description of the Prior Art
Wavelength Division Multiplexing (WDM) communication is one type of optical communication systems which transmit optical signals by using optical fibers. This WDM communication is a communication system which transmits multiple optical signals of different wavelengths using a single optical fiber. Multiple optical signals of different wavelengths are also called WDM signals. In many cases, each optical signal in WDM signals is counted, for example, in ascending order of wavelength (that is, starting at the shortest wavelength) as channel 1, channel 2, etc.
The spectrometer is a measuring equipment that obtains the wavelength spectrum of the light being measured (hereafter called ‘measured light beam’) using a wavelength dispersion device, determines the optical power existing in an arbitrary wavelength band, and measures the characteristics of the measured light beam using this determined optical power. This spectrometer is used for measuring WDM signals very frequently, and obtains the wavelength spectrum of input WDM signals and determines optical signal levels and wavelengths and the like for each channel using the optical power determined.
FIG. 1 is a configuration drawing indicating an embodiment of spectrometers that measure such WDM signals. In FIG. 1, spectroscope 10 is called a polychromator system into which WDM signals, the measured light beams, are input and which sends out the output corresponding to an optical power existing in an arbitrary wavelength band as a measured signal.
Spectroscope 10 is composed of optical fiber 11, collimating lens 12, grating 13 that is a wavelength dispersion device, focusing lens 14, mirror 15, and photodiode array module 16 (hereafter abbreviated as “PDM”).
Optical fiber 11 is a transmission line for making the measured light beam incident to spectroscope 10. Collimating lens 12 is installed counter to the optical output window of optical fiber 11 and emits measured light beam 100 output from optical fiber 11 after collimating it.
Grating 13 is installed oblique to collimating lens 12 to diffract the emitted light beam from collimating lens 12 by a desired angle. Then, grating 13 emits measured light beam 100 into a spectrum deflecting the light beam to different angles for every wavelength. Focusing lens 14 is provided on the optical path of emitted light from grating 13 and focuses the emitted light. Mirror 15 is installed to reflect the emitted light from focusing lens 14 in the desired direction.
PDM 16 is placed in the position at which measured light beam 100 reflected from mirror 15 focuses. On PDM 16, a PD array is formed, in which a plurality of strip-type or spot-type photodiodes (hereafter abbreviated as “PD”) is arranged. Each of these PDs generates a current (photocurrent) corresponding to the optical power of incident measured light beam 100 and outputs these photocurrents as measured signals of spectroscope 10.
In addition, a wavelength is assigned to each PD in advance. The assignment of wavelength corresponds to each position at which measured light beam 100 is deflected for each wavelength by grating 13 and focused on the PD array.
Control unit 20 comprises driving means 21, memorizing means 22, and calculating means 23. Driving means 21 changes over connections to each PD of PDM 16 in turn, reads measured signals from each PD in turn, for example, in ascending order of wavelength from the shortest one, and outputs each measured signal after converting them to the desired signals. Memorizing means 22 stores signals output from driving means 21 in turn. Calculating means 23 reads signals stored in memorizing means 22, determines the optical signal levels, wavelengths, or the like of measured light beam 100, and outputs the calculated results.
Operation of the spectrometer shown in FIG. 1 will now be described. Assume that different wavelengths of wavelength A and wavelength B are multiplexed in measured light beam 100. Measured light beam 100 emitted from optical fiber 11 is collimated with collimating lens 12. Measured light beam 100 transmitted through collimating lens 12 is incident to grating 13, and is spectrally divided into measured light beams 100A and 100B for each wavelength of λA and λB with this grating 13. Although measured light beams 100A and 100B spectrally divided with grating 13 are focused on the PD array of PDM 16 by focusing lens 14 and mirror 15, the positions of focusing the optical spot are shifted corresponding to wavelengths λA and λB of measured light beams 100A and 100B. Photocurrents are output from each PD respectively. As described above, spectroscope 10 does not contain mechanical moving parts and can operate stably for a long time.
Driving means 21 changes over connections to each PD of PDM 16 in turn, reads photocurrents generated in each PD in turn starting at the shortest wavelength, and converts these read photocurrents to voltages. In addition, since the signals converted to voltages are analog signals, driving means 21 converts these analog signals to digital signals and stores them in memorizing means 22 in the order of reading from each PD. Calculating means 23 determines the optical signal levels and peak wavelengths of each channel using digital signals stored in memorizing means 22 and wavelengths assigned to each PD, and outputs these calculation results. The output unit not shown displays the calculation results output from calculating means 23, for example, on the screen of the display unit or outputs them to external equipment not shown.
Subsequently, the action of calculating means 23 for determining the optical signal levels and peak wavelengths of each channel will now be described in detail. FIG. 2 schematically shows that part of the PD array is irradiated with measured light beam 100A. In FIG. 2, PD16a to PD16e are arranged in the direction in which measured light beam 100 is spectrally divided for wavelengths λA and λB by grating 13. Wavelengths of λa to λe (λa<λb< . . . <λe) are assigned to each PD of 16a to 16e respectively.
In addition, the PD array is not formed such that PD16a to PD16e that generate photocurrents are arranged without gaps in the direction of arrangement, but is formed such that PD16a of width Δp, a dead zone of width Δq, PD16b of width Δp . . . are arranged in this order in the direction of arrangement. Therefore, the width of one pitch is the sum of the width Δp of each PD of PD16a to PD16e and the width of dead zone Δq. Although each of PD16a to PD16e has the width Δp, the center positions of each PD in the direction of arrangement are generally made to correspond to assigned wavelengths λa to λe respectively.
From one side or both sides of each of PD16a to PD16e, photocurrents are output by signal wires not shown.
If measured light beam 100A has a line spectrum such as laser light, the optical spot of measured light beam 100A formed on the PD array takes the shape of an ellipse or circle, whose optical power shows a Gaussian distribution. In this case, it is assumed that the center of measured light beam 100A is in the vicinity of PD16c. FIG. 3 indicates the outputs of each of PD16a to PD16e stored in memorizing means 22. The abscissa shows wavelengths λa to λe assigned to each of PD16a to PD16e, and the ordinate shows the relative outputs of PD16a to PD16e. The outputs of PD16a to PD16e are represented by Pa to Pe. Since the center of measured light beam 100A exists in the vicinity of PD16c, it is apparent that the output Pc from PD16c is larger than any of the other outputs Pa, Pb, Pd, and Pe. In addition, Δλ shows a value in wavelength converted from the width of one pitch of the PD array.
The response of spectroscope 10 to a line spectrum input to it is approximated as a Gaussian distribution and the peak wavelength λpeak of measured light beam 100A can be expressed by equation (1).λpeak=λ0+δλ  (1)where λ0 is the wavelength λc assigned to PD16c whose optical power is closest to the peak optical power and δλ represents the difference between the peak wavelength λpeak and the wavelength λc assigned to PD16c whose optical power is closest to the peak optical power. The value δλ can also be expressed from equation (2) using the distance δx between the center of PD16c and the center of the optical spot of measured light beam 100A in FIG. 2, and the ratios of the output of PD16c nearest to the center of the optical spot of measured light beam 100A to each output of PD16b and PD16d both adjacent to PD16c.                                                                         δ                ⁢                                                                  ⁢                λ                            =                            ⁢                              δ                ⁢                                                                  ⁢                x                ⁢                                  Δλ                                                            (                                                                        Δ                          ⁢                                                                                                          ⁢                          p                                                +                                                  Δ                          ⁢                                                                                                          ⁢                          q                                                                    )                                        )                                                                                                                          =                            ⁢                                                                    Δ                    ⁢                                                                                  ⁢                    λ                                    2                                ·                                                      ln                    ⁡                                          (                                                                        P                                                      +                            1                                                                                                    P                                                      -                            1                                                                                              )                                                                            ln                    ⁡                                          (                                                                                                    P                            0                                                    ·                                                      P                            0                                                                                                                                P                                                          -                              1                                                                                ·                                                      P                                                          +                              1                                                                                                                          )                                                                                                                              (        2        )            where P0 corresponds to the output Pc of PD16c nearest to the peak optical power, and P−1 and P+1 correspond to Pb and Pd respectively.
The optical signal level Psig of measured light beam 100A can be determined as shown in equation (3) using the integral of the spectrum spread over the PD array, or the sum of output values Pb, Pc, and Pd from three PDs, that is, PD16b, PD16c, and PD16d near the peak optical power:Psig=α(δx)·(P−1+P0+P+1)  (3)where α(δx) is a function taking the distance between the center of the optical spot and the center of PD16c as a variable. This is because the value to be added differs depending on the distance between the center of the optical spot and the center of PD16c. This is a function determined by the diameter of the optical spot and the width of one pitch of the PD array.
Since operations in which calculating means 23 determines the optical signal level and peak wavelength of measured light beam 100B in the other channel are similar to the above, description of them will be omitted.
The wavelength resolution of spectroscope 10 depends on the size of the optical spot formed on the PD array. To improve the wavelength resolution, it is sufficient to make the optical spot size smaller (to sharpen the response spectrum) and focus it to one pitch of the PD array or less.
FIG. 4 shows the outputs of PD16a to PD16e, Pa to Pe, in the case of, for example, improving the wavelength resolution by taking the optical spot size to about one pitch of PD16a to PD16e. The wavelength resolution represents the performance that can identify channels if each channel is brought near. In FIG. 4, the same objects as those in FIG. 3 are given the same signs and so description of them is omitted.
FIG. 4 (a) indicates the case where the peak optical power of measured light beam 100A exists close to the center of PD16c. In FIG. 4 (a), outputs Pb and Pd that can be detected with PD16b and PD16d both adjacent to PD16c which is nearest to the peak become extremely small. For this reason, these are easily subjected to influences of noise and it is hard to determine the optical signal level and the peak wavelength accurately.
Also, FIG. 4 (b) indicates the case where the peak optical power of measured light beam 100A exists at about the mid point between PD16c and PD16d (dead zone). In FIG. 4 (b), since the major part of the optical power is concentrated in the dead zone, PD16c and PD16d, the output Pb that can be detected with PD16b becomes extremely small. For this reason, the output Pb is easily subjected to influences of noise and it is hard to determine the optical signal level and the peak wavelength accurately.
As described above, when the optical spot is made small to improve the wavelength resolution, the outputs of PDs to be used for calculation become small and are easily subjected to influences of noise. Accordingly, it becomes difficult to measure the optical signal level and the peak wavelength accurately. To reduce the influences of noise, it is sufficient to make the pitch of the PD array small. However, the types of generally available PD arrays are limited and it is not easy to change the shape such as changing the pitch of a PD array.